Walk me through how multiple users are looking at the same screen. Isn't everyone projecting 2 images of their own? Are the polarized glasses really narrow band, or something?
The retro-reflective surface makes sure the projector light only bounces back in your direction (well most of it).
Someone standing a few feet away won't see what you're seeing.
Without a multi-user frame sync, this prohibits users from being near-angles from each other to the retro-reflector, which basically prevents someone from "pointing" in AR into the others' field of view. Not necessarily a big limitation, but say you want to point out e.g. a terrain feature. If the software lets you select it, awesome! but if it's unselectable, you just have to try to describe it "the tree three trees over from the west wall... no not that one". Having good 'pointer drop' support mitigates this I suppose.
You can see the person's finger (or the CastAR wand's tip). I imagine the rendering engine will show the same thing in the same place relative to the real world for both users. It doesn't have to, in the case of a game where you each have your own point of view, but it could if the game works well that way.
The extent to which retro-reflective material reflects light back is actually quite incredible. I got a new bike light which for weeks I thought was REALLY REALLY BRIGHT until I realised that although it was making street signs 200m away flash brightly, it was only making them flash for me. Even a few degrees out and the reflectivity dropped remarkably.
